Automatic grinding machine

ABSTRACT

A machine for grinding an object, such as a printed circuit board coupon, having a region to be exposed and having a novel control track that has at least a portion of the rear edge of the track precisely located with respect to a predetermined grinding depth for the region is provided according to this invention. The grinding machine includes means for engaging the object with an abrasive device and means, interconnected with the control track on the object, for disengaging the object and the abrasive device when a break in the track has been ground.

FIELD OF INVENTION

This invention relates to a machine for automatically grinding an objecthaving a region to be exposed, and more particularly to such a grindingmachine which establishes an electrical circuit through a grinding guidefor one or more printed circuit board coupons to automatically andaccurately grind the coupons.

BACKGROUND OF INVENTION

Presently, grinding operations are controlled primarily by establishingtiming intervals for each application of an abrasive to an object to beground and by human observation of the rate at which the abrasives wearaway the object being ground. In another control procedure, a grindingoperator attempts to set mechanical stops at a level corresponding withthe desired grinding depth for each grinding operation.

One such object to be ground is a coupon for a printed circuit board.Printed circuit boards provide the mounting surface and electricalinterconnection system for components such as diodes, resistors andcapacitors. While originally the circuit boards were only printed on asingle side, most circuit boards today are double-sided or multilayered.Multilayer printed circuit boards, that is, circuit boards havingcircuits that are printed on several layers of the boards, requireuniform, dependable electrical connections between the layers. Thesecritical connections are typically provided by plated-through holesprepared by drilling holes through the stacked board layers. The drilledholes are plated with copper and then solder to establish the electricalinterconnections. Each and every plated-through hole must maintain apredetermined, uniform thickness: pores, cracks, nodules and otherfaults in plating render the entire circuit board defective.

A number of printed circuit boards are typically defined on a singlepanel of material. Fortunately for inspection purposes, plating faultstypically appear in most of the plated-through holes of a printedcircuit board if any faults are present at all. Therefore, one or morecoupons are defined on the panel for each circuit board to be printed.Each coupon has one or more test holes which are plated simultaneouslywith the plated-through holes. The coupons are labelled with a specificidentification code for the individual board and are detached for laterexamination.

The test holes in the coupon are typically aligned with theircenterlines in a plane perpendicular to the direction of grinding. Theyare examined after plating by accurately grinding one edge of the couponto expose the test holes in cross-section. However, the greater thedistance of the cross-section from the centerline, the greater is theerror in measurement of actual plating thickness.

Unfortunately for inspection purposes, grinding is presently a tedious,time-consuming process since great accuracy is demanded. At least threeor four steps are involved which are accomplished manually orsemi-automatically by human supervision of a grinding machine.Typically, one or more coupons are mounted in a holder with one edgeprotruding. The holder is cast in a mold after the coupons are carefullyarranged in the mold. The coupons are aligned in the mold usingalignment pins which pass through tooling holes in the coupon. Pottingmaterial is then poured into the mold which hardens to form the holder.Grinding machines accept one or more holders in a disk which is equippedwith a number of adjustable mechanical stops including hardened materialsuch as diamond.

The protruding edge of each coupon is ground for several minutes againstcoarse grit rotating at several hundred rpm. The coarse grit is replacedwith medium grit, which is rotated against the coupons for an additionalone to two minutes. When a semi-automated machine is used, the operatorremoves the coupons from the medium grit after all diamond stops contactthe grit; the operator then reset the diamond stops. Fine grit is thenapplied against the coupons for thirty to fifty seconds or until all thediamond stops again contact the abrasive. For the grinding machine, thediamond stops are reset flush with the holder. Finally, the coupons withtheir test holes exposed in cross-section are treated in one or morepolishing steps.

The sheer cost and labor of grinding one or more coupons per circuitboard present serious problems in view of the ever-increasing millionsof printed circuit boards that are produced annually. Presently, mostcoupons are ground manually at the cost of $15-20 per coupon. Manualgrinding requires constant operator attention and frequent inspectionusing a microscope. Each visual inspection interrupts the grindingoperation. The dependability of the operator varies greatly:overgrinding and undergrinding occur frequently. Since the coupons areexposed destructively, a mistake in overgrinding is irreparable andresults in the complete invalidation of the matching printed wire boardunless a second coupon is available for complete regrinding.Undergrinding, when detected, is cured by returning the coupon to theoperator, who must remount it and commence additional, unscheduledgrinding.

Semi-automated or machine-assisted grinding also requires close operatorattention. The machines are more dependable than grinding by hand butare not reliable for exposing small test holes due to cumulative sourcesof error. The tooling holes in one of the coupons can be misplacedrelative to the plated-through holes to be examined which results inunder- or overgrinding of the coupon. Further, the holder can beinitially misaligned in the disk relative to the setting of themechanical stops, and the stops themselves can wear over time. Also,several holders can be misplaced in a disk relative to each other.

SUMMARY OF INVENTION

It is therefore an object of this invention to provide an improvedgrinding machine in which the control of a grinding operation is fullyautomatic.

It is a further object of this invention to provide such a grindingmachine which is highly accurate.

It is a further object of this invention to provide an improved machinefor grinding printed circuit board coupons.

It is a further object of this invention to provide such a grindingmachine that consistently prepares the coupons so as to facilitateprecise measurement of the plating thickness of test holes.

A still further object of this invention is to provide such a grindingmachine which does not invalidate coupons due to overgrinding.

Yet another object of this invention is to provide such a grindingmachine which automatically conducts a number of grinding operationswithout human intervention.

This invention results from the realization that a truly effectiveautomatic machine for grinding objects such as printed circuit boardcoupons can be achieved by an apparatus which electrically interconnectswith a control track precisely located on the coupon and engages thecoupon with an abrasive. The apparatus disengages the coupon and theabrasive when a break in the track has been ground.

This invention features a machine for grinding an object having a regionto be exposed and having a control track that has at least a portion ofthe rear edge of the track precisely located with respect to apredetermined grinding depth for the region. There are means forengaging the object with an abrasive device and means, interconnectedwith the control track on the object, for disengaging the object and theabrasive device when a break in the track has been ground.

In one embodiment, the means for engaging includes means for contactingthe object to the abrasive device in an extended position and separatingthe object from the abrasive device in a retracted position. The meansfor engaging further includes first drive means for driving the meansfor contacting to the extended position and the means for disengagingincludes second drive means, responsive to sensing means for detecting abreak in the control track, for driving the means for contacting to theretracted position. The means for disengaging includes means forinterconnecting with at least two conductive leads responsive to thecontrol track. The means for contacting may include means for rotatingthe object and the means for disengaging may include means for rotatablyconnecting, such as slip ring means, with the means for interconnecting.The object may be a printed circuit board coupon having a test hole tobe exposed.

In a preferred embodiment, the means for contacting includes adouble-acting piston and the means for engaging includes rail means forguiding the means for rotating between the extended position andretracted position. The means for engaging further includes a cylinderwhich slidably carries the double-acting piston and the cylinderincludes a first inlet port proximate the end of the cylinder forreceiving the piston in a retracted position. The first drive means mayinclude engagement regulator means for controlling fluid delivery to thefirst inlet port to provide sufficient fluid pressure to engage theobject with the abrasive device until a break in the track is detected.The second drive means may include spring means for biasing the objectaway from the abrasive device.

In another embodiment, the cylinder further includes a second inlet portproximate the end of the cylinder opposite the first inlet port and thesecond drive means includes lift regulator means for controlling fluiddelivery to the second inlet port to provide sufficient fluid pressureto separate the object from the abrasive device. The means fordisengaging further includes exhaust valve means connected with thecylinder proximate the first inlet port. The engagement regulator meansovercomes the fluid pressure of the lift regulator means until the sensemeans actuates the exhaust valve means. The engagement regulator meansincludes solenoid valve means actuatable by the sense means to halt airdelivery to the first inlet port and to activate the exhaust valvemeans. The means for disengaging may include selector means forsuccessively monitoring a plurality of control tracks.

This invention also features a machine for grinding an object having aregion to be exposed and having a plurality of control tracks, eachtrack having at least a portion of its rear edge precisely located withrespect to a successive predetermined grinding depth of the region. Thegrinding machine includes an abrasive device, means for engaging theobject with the abrasive device, selector means for successivelymonitoring the control tracks, and means, interconnected with thecontrol track being monitored on the object, for disengaging the objectand the abrasive device when a break in the track being monitored hasbeen ground. The abrasive device may include means for rotatablyapplying an abrasive to the object and the means for engaging caninclude means for rotating the object in a direction opposite to themeans for rotatably applying. The abrasive device may include aplurality of abrasive wheels.

The invention may also be expressed as a machine for grinding aplurality of printed circuit board coupons, including an abrasive devicehaving a plurality of abrasive wheels and a plurality of grinding heads.Each grinding head includes means for engaging the coupon with one ofthe abrasive wheels, selector means for successively monitoring thecontrol tracks of a coupon, and means for disengaging the coupon and theabrasive device when a break in the track being monitored has beenground.

DESCRIPTION OF PREFERRED EMBODIMENTS

Other objects, features and advantages will occur from the followingdescriptions of preferred embodiments and the accompanying drawings, inwhich:

FIG. 1 is an exploded axonometric view of a conventional multi-layerprinted circuit board;

FIG. 2 is a cross-sectional view of an exposed plated-through hole inthe multilayer circuit board of FIG. 1 illustrating potential faults inthe plating operation;

FIG. 3A is a schematic top plan view of conventional coupons and circuitboards disposed on a panel;

FIG. 3B is an enlarged view of one of the coupons of FIG. 3A;

FIG. 4 is a chart of error arising during measurement of platingthickness relative to the distance of the cross section from thediameter of the test hole;

FIG. 5A is a top plan view of a novel grinding guide that is printed ona coupon;

FIG. 5B is an enlarged detailed view of a portion of a control track ofFIG. 5A showing its following edge aligned by a drilled hole;

FIG. 6 is an axonometric view of the coupon of FIG. 5A embedded in acoupon mount;

FIG. 7A is an axonometric view of a grinding machine according to thisinvention for accepting the coupon mount of FIG. 6;

FIG. 7B is a schematic block diagram of the grinding machine of FIG. 7Ashowing electrical circuits and pneumatic pathways;

FIG. 7C is a schematic of the electrical circuits of FIG. 7B;

FIGS. 8A-8C are schematic cross-sectional views of a multilayer couponcontaining a grinding guide similar to the guide of FIG. 5A aftercoarse, medium and fine grinding, respectively;

FIG. 9 is a top plan view of control tracks which have their back edgesprecisely located by alternative arrangements;

FIG. 10 is a top plan view of yet another grinding guide;

FIG. 11 is a schematic axonometric view of an alternative, multi-stationgrinding machine according to this invention; and

FIG. 12 is a schematic of the grinding operation selector, andassociated controls for each grinding head of the multistation grindingmachine of FIG. 11.

While a grinding machine according to this invention can be used togrind objects such as coupons for single- or double-sided circuitboards, the grinding machine is particularly useful for the grinding ofcoupons for multilayer circuit boards, such as shown in FIG. 1, since agreat deal of time and money is invested in each multilayer board.Multilayer printed circuit board 10 includes layers 12, 14, 16, 18, and20. Circuit board layers 12, 16, and 20 are printed on both sides toplace copper cladding on those layers to form the circuits. The circuitsare insulated from each other by insulating layers 14 and 18,respectively. Circuits printed on the upper sides of board layers 12,16, 20 are shown as solid lines while the printed circuits on theunderside of these layers are indicated by open lines.

After assembly and alignment of board layers and the insulating layersrelative to each other, the boards are selectively plated to establishplated-through holes to complete the circuits; holes 26, 28, shown inphantom, represent the electrical interconnection provided by theplated-through holes. Components on mounts 30, for example, areinterconnected to other components by lines 32, 34, 36, and 38 whenholes 40, 41, 42, 43 and 44 in their respective layers are plated as asingle plated-through hole as illustrated by hole 26.

Plated-through hole 26 of assembled multilayer circuit board 10, FIG. 1,is shown in cross-section in FIG. 2 to illustrate possible faults risingduring the final plating-through stage of manufacture; these faults willhopefully be discovered by examining test holes in accompanying coupons.Printed circuit boards 12, 16 and 20 are shown with upper and lowercopper cladding 30 and 32, 34 and 36, and 38 and 39, respectively. Thecircuits are separated internally by insulating layers 14, 18 unless aplated-through hole such as hole 26 electrically connects thesecircuits. After the layers are assembled together, an additional platingstep plates copper cladding 90 through hole 26. Artifact 91 represents aresin smear that was not properly removed and which diminishes theelectrical connection between cladding 36 and plating 90. Voids 92 inthe laminate indicate a weakened structure and the potential for shortcircuits between copper cladding circuits. Channel 93 is a hairlinecrack in plating 90.

Solder plating 94 is then plated over copper plating 90. Defect 96indicates discontinuous plating while void 98 illustrates a void in theplating penetrating through both copper plating 90 and solder plating94. Channel 99 represents a large crack which penetrates copper plating90.

While some defects such as nodule 100 are acceptable, copper plating 90may develop an unacceptable nodule 102. Also unacceptable are platingpocket 104 and circumferential cracks 106, 108.

Finally, the platings themselves have specified thicknesses. Dimensionarrows 110 indicates the range of desired plating thickness for solderplating 92, which is typically 0.001 to 0.002 inch. Dimension arrows 112represents that the range of acceptable widths for copper plating 90 is0.001 to 0.002 inch.

The need to determine the quality of plating-through is readilyapparent. Also apparent is that individual electrical testing of eachand every plated-through hole is prohibitively expensive and timeconsuming. Further, destructive testing best exposes the defects yetcannot be performed on the printed circuit boards themselves. Separatecoupons having a number of sacrificial test holes therefore provide apractical mechanism for ascertaining the plating quality.

Typically, a number of circuit boards are printed on a single panel suchas panel 46, FIG. 3A. Printed wire boards 48, 50, 52, and 54 have couponpairs 56, 58, 60 and 62 located in close proximity on panel 46 to theirrespective printed wire boards. To test the quality of printing, lines68 on coupon 69 are later stressed to assure proper plating. To test thequality of plated-through holes such as holes 64 in printed wire board48, test holes on coupon 66 of coupon pair 56 are subsequently examinedin cross-section.

Coupon 66 is shown in greater detail in FIG. 3B. Typically, test holes70 and 72 are drilled before holes 64 are drilled and test holes 74 and76 are drilled afterward to monitor the quality of the drill bitstarting and finishing work on board 48. Test holes 70, 72, 74 and 76are plated through simultaneously with holes 64.

Also shown are alignment holes 80, 82 used for conventionally aligningthe coupons in a mold before potting material is added to form a holderwhich carries the coupon. Coupon 66 is removed from panel 46 usingshears or a punch.

When test holes in a coupon are exposed in cross-section, the depth ofexposure affects the measurement of plating thickness. FIG. 4 is a chartof error arising during the measurement of the thickness of copperplating 113 relative to the distance of the cross section from thediameter of plated-through hole 72, FIG. 3B. Line 114 represents a crosssection corresponding with the diameter while lines 116, 118, 120 and122 represent cross sections of exposure which are increasinglydisplaced from diameter 114. The measurement error with respect todisplacement from diameter 114 in mils is shown in Table I when hole 72is 16 mils and plating 113 is 1 mil.

                  TABLE I                                                         ______________________________________                                        MIL         MEASUREMENT                                                       DISPL       ERROR (mil)                                                       ______________________________________                                        1.0         0.0250                                                            2.0         0.0625                                                            3.0         0.1000                                                            4.0         0.1750                                                            ______________________________________                                    

Cross section 116 is displaced 0.001 inch--1 mil--relative to line 114.The difference in length between radius line 124 within copper plating90 and cross section 116 is 0.025 mil. Cross section 122 overestimatesthe true thickness represented by line 126 by 0.175 mil, which is a 17.5percent measurement error.

Such errors from overgrinding and undergrinding can be prevented by agrinding machine according to this invention which uses a novel grindingguide, e.g., grinding guide 130 as shown in FIG. 5A. Grinding guide 130is mounted on coupon 132. Tooling holes 134, 136, shown in phantom, arenot part of grinding guide 130 but are utilized during conventionalhandling and alignment of coupon 132. Grinding guide 130 includesconductive leads 138, 140, 142, and 144. Conductive leads 138 and 140are interconnected by conductive control track 146, leads 142 and 144are connected by track 148, and leads 140 and 142 are connected by track150. Control holes 152, 154 and 156 define tracks 146, 148 and 150 astracks which are successively broken during three grinding steps such ascoarse, medium and fine grinding. While the control holes are shown asdrilled holes, track material encompassed by hole 152, for example, canbe removed by laser etching or other removal method.

Electrically conductive leads 138, 140, 142 and 144 are shown associatedwith plated-through test holes 160, 162, 164 and 166. As surface 168 ofcoupon 132 is being ground to expose those test holes in cross section,a break occurs first in track 146 as the grinding erodes material up tocontrol hole 152. A circuit is formed by providing power to lead 140, oranother lead responsive to track 150, and sensing current or voltage onlead 138. The circuit is broken when surface 168 is ground through track146 to control hole 152. A predetermined grinding depth is therebyestablished by that control track.

After track 146 is breached, a lesser abrasive is applied to grindingsurface 168 until track 148 is broken. Finally, grinding with a fineabrasive continues until track 150 is broken. When it is desirable tomaintain positive power on a single lead rather than transferring powerto successive leads as successive tracks are monitored, track 140 or 142can be designated as a power lead and the remaining three tracks can bemonitored for sudden drops in power indicating a broken track.

Conductive leads 140, 142 and control track 150 are shown in an enlargedview in FIG. 5B. Control hole 156 is shown in relation to diameter 170passing through the centers of test holes 162, 164. Dimension 172illustrates that control hole 156 precedes diameter 170 by a smallamount to ensure that the grinding machine conducting the grindingoperation has sufficient time to retract coupon 132 from the abrasiveand to allow some of surface 168 to be further eroded during polishing.For typical coupon grinding operations, the anticipation distancerepresented at 172 is 1 to 2 mil. Control track 154, FIG. 5A,anticipates the diameter of the test holes by 3 mil and coarse controltrack 152 anticipates by 6 mil.

To prepare coupon 132 for grinding, it is "potted" in potting materialsuch as Epo-Kwik epoxy, available from Buehler, which hardens to formcoupon mount 174, FIG. 6. Coupon 132 is surrounded by potting materialup to line 176. During grinding, surface 168 of coupon 132 is grounddown to surface 168a, indicated in phantom.

Conductive leads 138, 140, 142 and 144 are shown slightly raised inthickness so that they project somewhat from the surface of coupon 132.The projection facilitates mating with an edge connector of a grindingmachine according to this invention as described below.

While individual grinding control per coupon is most accurately obtainedby grinding a single coupon at a time, two or more coupons can bemounted in the same mount as indicated by coupon 178, shown in phantom.Additional coupons are aligned during potting relative to coupon 132,for example. It is desirable for the upper portion of each non-monitoredcoupon 178 not to extend far above upper potting material limit 176:physical interference with the interconnection of monitored coupon 132and a grinding machine according to this invention is thereby avoided.Coupon 132 is then monitored during grinding to control the grinding ofall coupons in mount 174.

One example of a machine according to this invention for grindingobjects provided with grinding guides is shown in FIG. 7A. Grindingmachine 180, attached to the upper surface of table 181, pneumaticallyoperates a double-acting piston within cylinder 182 to extend andretract slide assembly 184. Slide assembly 184, shown in the retractedposition, has holder 186 for securing coupon mount 176.

In the extended position, the coupon in mount 174 is contacted toabrasive wheel 190. Abrasive wheel 190 is rotated in one direction byabrasive motor 192 at a rate controlled by abrasive speed control 193.Smearing of the surface being ground is minimized by rotating thecoupons in the same or opposite direction. To increase the effectiverate of grinding, the coupon or coupons in coupon mount 174 are rotatedin the opposite direction by motor 194. Motor 194 exerts torque onholder 186 through gear box 196 and belt-driven shaft 188.

To reach the extended position, slide assembly 184 is driven along rail198 as determined by a balance of pressures controlled by engagementregulator 200 and lift regulator 208 (not visible). Air at a pressure ofat least 50 psi is supplied through hose 202 to grinding machine 180 andis filtered by filters 204, 206. As shown in FIGS. 7A and 7B, air passesthrough lift regulator 208 which controls the pressure of the airentering the lower portion of cylinder 182. Lubricator 210 provides afog of lubricant to the air entering cylinder 182. Air passes fromengagement regulator 200 through lubricator 212 (not visible in FIG. 7A)and is conducted through solenoid valve 214 to quick exhaust valve 216which is connected to the upper portion of cylinder 182. Engagementregulator 200 is adjusted to pass air at a sufficient pressure toovercome the lift pressure regulated by lift regulator 208. Slideassembly 184 is thereby driven from the retracted to the extendedposition to engage the coupons against abrasive wheel 190.

During operation, slide assembly 184 is biased toward grinding wheel190, FIG. 7A, to abraid coupons in mount 174. For wet grinding, water issupplied through faucet 218. Shaft 188 is rotated in a directionopposite to the direction of wheel 190 by motor 194, FIGS. 7A and 7B, ata rate measured by tachometer 220 and controlled by motor torquecontroller 222. The grinding guides on one or more coupons within mount174 are interconnected with sensors in grinding machine 180 whichmonitor voltage or current supplied by guide power supply 225. A breakin the control track being monitored trips relay 224 which in turncloses solenoid valve 214. When pressure on the intake portion ofexhaust valve 216 ceases, exhaust valve 216 opens to quickly ventpressure in the upper portion of cylinder 182. This allows air pressureprovided by lift regulator 208 to quickly drive slide assembly 184 tothe retracted position thereby separating the coupons from abrasive 190.Overgrinding or undergrinding of the coupons is thereby avoided.

The housing containing motor torque controller 222 is shown in FIG. 7Ahaving speed control 226, slide lift 228, grinding selector 230, andmain power line 232. The electrical circuits and pneumatic pathways ofgrinding machine 180 are described further in relation to FIGS. 7B and7C. As shown in FIG. 7B, grinding guide 130 on coupon 132 electricallyinterconnects with connector 240, such as a conventional edge connector,which is in turn rotatably interconnected to slip ring 242. After theappropriate control track of grinding guide 130 has been selected bygrinding selector 230, power from guide power supply 225 passes throughslip ring 242 to connector 240 and coupon 132. As long as the controltrack on coupon 132 remains intact, an energizing circuit is maintainedthrough relay 224. When the control track is ground through, theenergizing circuit is broken which trips relay 224 and closes solenoidvalve 214. Exhaust valve 216 opens and piston 234 experiences a suddendecrease in pressure on its upper end. The piston rises rapidly,bringing with it slide assembly 184 which is attached to piston shaft236 by linkage 238.

The energizing circuit can also be broken manually by slide lift switch228. Alternatively, the energizing circuit can be maintained for adesired period of time through polish line 244.

The electrical circuits of FIG. 7B are shown in more detail in FIG. 7C.Speed control 226 includes potentiometer 246. Grinding operationselector 230a successively monitors three separate control tracks ongrinding guide 130. Switch 246 is set to COARSE during the firstgrinding operation. While slide lift switch 228 is set to AUTO, solenoidvalve 214 is not activated by relay 224 until the COARSE control trackis ground through. The coarse abrasive is then replaced with a mediumabrasive and switch 246 is set to medium setting MED. After the mediumtrack is broken, switch 246 is set to FINE to grind the test holes incoupon 132a to the desired grinding depth. When the FINE control trackis ground through, the fine abrasive is replaced with a polish andswitch 246 is held against POLISH, line 244, for thirty to sixtyseconds.

Examples of the different grinding depths achieved in a multilayeredcoupon are shown in FIGS. 8A through 8C. Multilayered coupon 132acarries conductive leads 138a, 140a, 142a, and 144a on one of its outersurfaces. A grinding operation, consisting of one or more grindingsteps, proceeds until control track 146a is ground through, that is,until control hole 152a is reached. A circuit previously establishedbetween conductive leads 138a and 140a through track 146a andplated-through test holes 160a, 162a is therefore broken. The width oftest hole 160a at this predetermined grinding depth is indicated bydimension 250, which is less than the true diameter of test hole 160a.Similarly, test holes 162a, 164a and 166a are also ground incross-section to less than their full diameters.

If control is desired not only over grinding depth but also overgrinding angle relative to the axes of the test holes, one or moreadditional sets of control tracks can be provided. For example, as shownin phantom, control tracks 146b, 148b and 150b are provided on layersother than the layers on which control tracks 146a, 148a and 150a aredisposed. Control tracks 146b, 148b, and 150b are electrically connectedto conductive leads 138a, 140a, 142a and 144a by plated-through testholes 160a, 162a, 164a and 166a.

After the grinding operation selector is reset to establish a circuitbetween conductive leads 140a and 144a, FIG. 8B, coupon 132a is appliedto medium abrasive until the rear edge of control track 148a is reachedat control hole 154a. Dimension 252 of test hole 160a is greater thandimension 250, FIG. 8A. Next, as shown in FIG. 8C, the cross-sectionalsurface of coupon 132a is ground until the circuit established betweenconductive leads 140a and 142a is broken when track 150a is groundthrough. A portion of control hole 156a is now revealed in crosssection. Dimension 254 of control hole 160a is nearly as great as itstrue diameter. Timed polishing is then conducted to reach the truediameters of the test holes and to facilitate microscopic examination ofthe test holes.

While the rear edges of the control tracks of the grinding guidesdescribed above have been set by precisely located control holes, thisis not a limitation of the grinding guides. Grinding guide 130b, FIG. 9,encompassing test holes 256, 258, 260 and 262, precisely locates therear edges of control tracks 264, 266, and 268 using differenttechniques. The rear edge of control track 264 is determined byrectangular notch 270 while that of track 266 is determined by angularnotch 272. Accordingly, any recess can be used to establish the rearedge of a control track. Alternatively, the entire control track can beprecisely laid perpendicular to the desired depth of grinding. Forexample, back edge 274 of control track 268 is precisely located withrespect to the widths of test holes 256, 258.

Test holes need not be directly associated with the conductive leads orthe control tracks of a grinding guide when leads directly interconectwith each track. Grinding guide 130c, FIG. 10, is mounted on coupon 132cseparate from test holes 256a, 258a, 260a and 262. During grinding, thefirst grinding operation is controlled by establishing a circuit throughcontrol track 268a. This may be accomplished by providing power toconductive lead 276 which is linked to conductive lead 278 throughcontrol tracks 264a, 266a and 268a. Alternatively, separate conductivelead 284, shown in phantom, can be provided such that control track 268ainterconnects power conductive lead 284 and monitored conductive lead278. The next grinding operation is controlled by the rear edge of track266a which interconnects conductive leads 280 and 276; alternatively,track 266a can interconnect tracks 286, shown in phantom, and conductivelead 280. The final grinding operation is controlled by track 264a whichinterconnects power conductive lead 276 and monitored lead 282.

Dashed line 288 illustrates that acceptable lengths of conductive leads278, 280, 282 and 276 are such that portions of these conductive leadsremain externally exposed after potting: line 288 represents anacceptable upper limit of the potting material which covers the testholes. Alternatively, the conductive leads can be located between thelayers of a multilayer coupon as long as a grinding machine according tothis invention is able to interconnect at a locus with each conductivelead.

While the grinding machine is described above as utilizing grindingguides for the grinding of printed circuit board coupons having testholes to be exposed, this is not a limitation of the invention. Thegrinding guides may be used to control grinding by machines according tothis invention of any object to be ground. The control tracks of thegrinding guide for the object can be precisely located duringmanufacture of the object or can be applied afterward. For integrated orhybrid circuits, the grinding guide can be applied simultaneously to acarrier material using a photolithographic process. The carrier materialcan be a metal or a ceramic; the region to be exposed can be anintegrated circuit chip capacitor or resistor. Alternatively, thecontrol track can be applied after manufacture as a foil tape.

Further, the grinding guide can be used to control the grinding ofobjects that do not include an electrical circuit. For example, theporosity of a ceramic can be determined by placing a foil tape controltrack on the ceramic and grinding until the track is breached: theexposed surface is then examined for pore size and density.

In another application, an intact casting is probed with X-rays orultrasound to locate voids or other defects to be studied. Informationprovided by the X-ray is used to determine the precise location for thecontrol track. The grinding guide is then directly applied to thecasting or the casting is potted in alignment with a grinding guidecarried by a separate object that is ground simultaneously with thecasting.

An alternative multi-station grinding machine according to thisinvention is shown in FIG. 11. Grinding machine 290 includes slideassembly 292, hereinafter referred to as grinding head 292, which movesbetween a retracted position and an extended position along track 294.Track 294, a type of rail, is mounted on channel 296 of turret 298.Tower 298 includes central aluminum tubing 300 on which are mountedchannel 296 and five other channels such as channels 302 and 304. Asdescribed below, turret 298 can host up to six grinding heads, one oneach channel.

Grinding machine 290 also includes base 306 which includes grindingwheels 308, 310, and 312 which contain coarse, medium, and fineabrasives, respectively. A portion of base 306 has been cut away toreveal abrasive motor 314 for wheel 308. Similarly, each abrasive wheelhas a separate motor. Base 306 also includes polish wheel 316 and sievebasin 318.

Cylinder 320, mounted on support 322, is pneumatically activated todrive grinding head 292 to the extended position. Flexible conduit 324such as a Gore tube conveys cables to grinding head 292. Cylinder 320contains a return spring which biases the coupon away from the abrasivein the extended position.

During operation, grinding head 292 engages an object such as a mountedcoupon on coarse abrasive wheel 308 until the coarse control track isground through. At that point, grinding head 292 retracts, driven by thereturn spring. After the spring in cylinder 320 drives grinding head 292to the retracted position, turret 298 is rotated by turret indexingmotor 326 as commanded by turret motor controller 328. The coupons heldby grinding head 292 are then passed through spray trough 330 as turret298 rotates. Spray trough 330 contains nozzles which direct water ontothe coupon mount to wash off grinding debris.

The coupons are then ground against medium abrasive wheel 310 until themedium control track is ground through. When the medium track is broken,grinding head 292 immediately retracts and then is passed through spraytrough 332 by indexing motor 326. A circuit is established through thefine control track and grinding head 292 is extended to engage fineabrasive wheel 312. After the fine control track is broken and grindinghead 292 retracts, the coupons are passed through spray trough 334 andthe coupons are engaged with polish wheel 316 for 40-50 seconds. Afterpassing through final spray trough 336, the coupons in their couponmount are ejected into sieve 318 for later removal.

To increase throughput of grinding machine 290 additional grinding headssuch as grinding head 338, shown in phantom, are added to turret 298.When multiple driving heads are used, it is desirable to add anadditional coarse abrasive wheel 340 and spray trough 342. The grindingoperation requires the longest time; by adding one or more additionalcoarse abrasive wheels, the grinding heads engaging those wheels can beretracted at approximately the same time as the other grinding heads areretracted. Each rotatable grinding head includes a solenoid, a quickexhaust valve, a cylinder and a slide assembly. To allow continualrotation, the air hoses and electrical connections supplying thesecomponents contain rotatable connectors within base 306.

The control for each grinding head of multi-station grinding machine 290is shown in FIG. 12. Grinding operation selector 230b includes switchindexer 344 which indexes switch 246a to control successive grindingoperations. During operation, loading of the coupon mount generatesloader cue signal 346 which passes through slip rings 348 to turretmotor controller 328. After all six grinding heads are in the retractedposition and the next coupon mount to be ground is loaded, turret motorcontroller 328 commands the selector switch indexer of each grindinghead to advance. For grinding head 292, for example, switch 246a ismoved from coupon eject 350 to OR gate 352. Slide lift switch 228a isset as shown. Referring to FIGS. 11 and 12, grinding head 292 thencontacts the coupon to abrasive wheel 340 until coarse timer 354 breaksthe energizing circuit after one to two minutes if the coarse controltrack has not been ground through by that time; either timer 354 orCOARSE can trip relay 224a. After all six grinding heads are retractedas commanded by their individual control circuits, and the grinding headcounterclockwise to grinding head 292 is loaded with a coupon mount,turret motor controller 328 activates indexing motor 326 to rotateturret 298 in a clockwise direction. Motor controller 328 commandsswitch indexer 344 to move switch 246a to COARSE which establishes asensing circuit through slip rings 242a with the coarse control track onthe coupon being monitored as described above. The control track is thenused to determine when grinding head 292 retracts from coarse wheel 308,FIG. 11.

Similarly, MED & FINE establish energizing circuits with the medium andfine control tracks, FIG. 12. When grinding head 292 reaches polishwheel 316, FIG. 11, the coupons are engaged with polish 316 for a lengthof time determined by polish timer 356, FIG. 12.

Although specific features of the invention are shown in some drawingsand not others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention.

Other embodiments will occur to those skilled in the art and are withinthe following claims.

What is claimed is:
 1. A machine for grinding a printed circuit boardcoupon having a test hole to be exposed and having a plurality ofcontrol tracks, each track having at least a portion of the rear edge ofthe track precisely located with respect to a predetermined grindingdepth for the test hole, comprising:means for engaging the coupon withan abrasive device; and means, interconnected with the control track onthe coupon, for disengaging the coupon and the abrasive device when abreak in the track being monitored has been ground, said means fordisengaging including selector means for successively monitoring thecontrol tracks.
 2. The grinding machine of claim 1 in which said meansfor engaging includes means for contacting the coupon to the abrasivedevice in an extended position and separating the coupon from theabrasive device in a retracted position.
 3. The grinding machine ofclaim 2 in which said means for engaging further includes first drivemeans for driving said means for contacting to the extended position. 4.The grinding machine of claim 3 in which said means for disengagingincludes sensing means for detecting a break in the control track. 5.The grinding machine of claim 4 in which said means for disengagingfurther includes second drive means, responsive to said sensing means,for driving said means for contacting to the retracted position.
 6. Thegrinding machine of claim 5 in which said means for disengaging includesmeans for interconnecting with at least two conductive leads responsiveto the control track.
 7. The grinding machine of claim 6 in which saidmeans for contacting includes means for rotating the coupon.
 8. Thegrinding machine of claim 7 in which said means for disengaging includesmeans for rotatably connecting with said means for interconnecting. 9.The grinding machine of claim 6 in which said means for interconnectingslidably, removably interconnects with said conductive leads.
 10. Amachine for grinding an object having a region to be exposed and havinga plurality of control tracks, each track having at least a portion ofthe rear edge of the track precisely located with respect to apredetermined grinding depth for the region, comprising:means forengaging the object with an abrasive device; and means, interconnectedwith the control track on the object, for disengaging the object and theabrasive device when a break in the track being monitored has beenground, said means for disengaging including selector means forsuccessively monitoring the control tracks.
 11. The grinding machine ofclaim 10 in which said means for engaging includes means for contactingthe object to the abrasive device in an extended position and separatingthe object from the abrasive device in a retracted position.
 12. Thegrinding machine of claim 11 in which said means for engaging furtherincludes first drive means for driving said means for contacting to theextended position.
 13. The grinding machine of claim 12 in which saidmeans for disengaging includes sensing means for detecting a break inthe control track.
 14. The grinding machine of claim 13 in which saidmeans for disengaging further includes second drive means, responsive tosaid sensing means, for driving said means for contacting to theretracted position.
 15. The grinding machine of claim 14 in which saidmeans for disengaging includes means for interconnecting with at leasttwo conductive leads responsive to the control track.
 16. The grindingmachine of claim 15 in which said means for contacting includes meansfor rotating the object.
 17. The grinding machine of claim 16 in whichsaid means for disengaging includes means for rotatably connecting withsaid means for interconnecting.
 18. The grinding machine of claim 17 inwhich said means for rotatably connecting includes slip ring means. 19.The grinding machine of claim 16 in which said means for engagingincludes rail means for guiding said means for rotating between theextended position and the retracted position.
 20. The grinding machineof claim 16 in which said means for contacting includes a double-actingpiston.
 21. The grinding machine of claim 20 in which said means forengaging includes a cylinder which slidably carries said piston, saidcylinder including a first inlet port proximate the end of said cylinderreceiving said piston in the retracted position.
 22. The grindingmachine of claim 21 in which said first drive means includes engagementregulator means for controlling fluid delivery to said first inlet portto provide sufficient fluid pressure to engage the object with theabrasive device until a break in the track is detected.
 23. The grindingmachine of claim 22 in which said second drive means includes springmeans for biasing the object away from the abrasive device.
 24. Thegrinding machine of claim 22 in which said cylinder further includes asecond inlet port proximate the end of said cylinder opposite said firstinlet port and said second drive means includes lift regulator means forcontrolling fluid delivery to said second inlet port to providesufficient fluid pressure to separate the object from the abrasivedevice.
 25. The grinding machine of claim 24 in which said means fordisengaging further includes exhaust valve means connected with saidcylinder proximate said first inlet port and in which said engagementregulator means overcomes the fluid pressure of said lift regulatormeans until said sense means actuates said exhaust valve means.
 26. Thegrinding machine of claim 25 in which said engagement regulator meansincludes solenoid valve means actuatable by said sense means to haltfluid delivery to said first inlet port and to actuate said exhaustvalve means.
 27. The grinding machine of claim 15 in which said meansfor interconnecting slidably, removably interconnects with saidconductive leads.
 28. The grinding machine of claim 10 in which saidmeans for engaging includes holder means for securing the object.
 29. Amachine for grinding an object having a region to be exposed and havinga plurality of control tracks, each track having at least a portion ofthe rear edge of the track precisely located with respect to asuccessive predetermined grinding depth of the region, comprising:anabrasive device; means for engaging the object with said abrasivedevice; selector means for successively monitoring the control tracks;and means, interconnected with the control track being monitored on theobject, for disengaging the object and said abrasive device when a breakin the track being monitored has been ground.
 30. The grinding machineof claim 29 in which said abrasive device includes means for rotatablyapplying an abrasive to the object.
 31. The grinding machine of claim 30in which said means for engaging includes means for rotating the object.32. The grinding machine of claim 31 in which said means for rotatablyapplying rotates said abrasive in a direction opposite to that of saidmeans for rotating the object.
 33. A machine for grinding a plurality ofprinted circuit board coupons, each coupon having a test hole to beexposed and having a plurality of control tracks, each track having atleast a portion of the rear edge of the track precisely located withrespect to a successive predetermined grinding depth of the test hole,comprising:an abrasive device having a plurality of abrasive wheels; anda plurality of grinding heads, each grinding head including:means forengaging the coupon with one of said abrasive wheels; selector means forsuccessively monitoring the control tracks of a coupon; and means,interconnected with the control track being monitored on the coupon, fordisengaging the coupon and the abrasive device when a break in the trackbeing monitored has been ground.